In prior U.S. Pat. No. 4,360,728, assigned to the assignee of the present invention, a banking card is disclosed suitable for use with automatic teller machines (ATMs). The patent describes a card having optical data written by a laser which forms pits in the medium. The medium itself is described as a thin metallic recording layer made of reflective metal. Types of reflective recording material suitable for use in such cards are described in U.S. Pat. Nos. 4,269,917; 4,304,848 and 4,363,870, all assigned to the assignee of the present invention. This material is characterized by reflective silver particles in a gelatin matrix. The silver particles form a reflective field which may be modified by laser writing or, in some instances, by photographic prerecording of information. Since the material described in the patents is based upon silver halide emulsions, exposure of data patterns and subsequent developing of the patterns leads to recording of data in a manner similar to laser writing. In either case, data is represented by spots or pits in a reflective field. Reflective spots in an absorptive field would also work.
In many optical data recording and retrieval systems, data has been recorded in linear tracks, analogous to magnetic recording. Data is read by following tracks, usually by means of servo systems. Attempts have been made to increase the data density of optical storage media without increasing the read error rate. Some lengthwise compression of data has been achieved in both magnetic and optical systems by means of clever modulation schemes, such as FM and MFM. In U.S. Pat. No. 3,980,818, Browning teaches data storage in a grid of perpendicular contiguous non-overlapping rows and columns. The data is read with a single detector in a raster scan-like manner.
In U.S. Pat. No. 4,105,926, Reno et al. teaches a transparent film containing indicia through which a light beam passes. A mirror behind the film reflects the beam so that it illuminates a small area of the film from behind. In the absence of opaque indicia at the illuminated spot, light is transmitted to a photodetector. The spots represent data bits with opaque spots representing ones and transparent spots representing zeros. In practice, there may be N parallel tracks and N respective photodetectors.
Unfortunately, a signal "crosstalk", i.e. detection of unwanted light from adjacent areas around an illuminated spot, often occurs when data spots and tracks are spaced too closely, due to the fact that some of the light from each track reaches photodetectors intended to receive light from other nearby tracks. In order to reduce crosstalk, the parallel tracks are typically not closely spaced and substantial blank space is provided between bit positions. Others, such as Browning, sacrifice speed in reading data for higher density by using only a single detector.
An object of the present invention was to devise a means for formatting optically recorded data for a data card, or the like, in a way that increases data density, while at the same time minimizes error resulting from crosstalk and other sources.